Hard time accepted by criminal up in nick (5)
I believe the answer is:
notch
'nick' is the definition.
(both can mean a small cut or impression)
'hard time accepted by criminal up' is the wordplay.
'hard' becomes 'h' (abbreviation used in pencil classifications).
'time' becomes 't' (abbreviation).
'accepted by' means one lot of letters goes inside another (inserted letters are accepted).
'criminal' becomes 'con' (short for convict).
'up' is a reversal indicator (in down clue: letters go upwards).
't' inserted inside 'con' is 'cton'.
'h'+'cton'='hcton'
'hcton' backwards is 'NOTCH'.
'in' is the link.
(Other definitions for notch that I've seen before include "small score?" , "Incision; indentation" , "Salad plant, garnish" , "Tallying incision" , "Gouge" .)